Remove $schema line from Builder schema

The $schema line was causing json-schema to fail validation. That line
doesn't have all that much useful information anyway.
Unify ShapeData schemas

Use union notation in JSON schemas to unify the circle and rectangle
schemas into one file.
Add JSON frontend schema file

Add schema for use in a web frontend.
Add simple JSON verification script
Move JSON validator to its own subproject

It has dependencies distinct from those of node-markup, so move it to
its own folder so I can have a custom install script in there.
Add tutorial file describing how to use JSON

Explain specification and include example strings for JSON and Markup
strings in frontend and backend scenarios.
Improve wording of circle draw instruction

The original wording didn't make much sense in the TUTORIAL.md
documentation.
Merge branch 'documentation'